1. Field of the Invention
The field of the invention is that of optical components and more particularly that of optical components which can be integrated into hybrid or monolithic optical or opto-electronic integrated circuits. To be more precise, the invention concerns a transition between different waveguide structures providing optical interconnections between different components of an optical or electro-optical circuit. The waveguide structures to which the invention relates are known in this art as ridged optical waveguides and buried optical waveguides.
2. Description of the Prior Art
Electro-optical components that may need to be connected by interconnect waveguide structures include, for example: lasers, light-emitting diodes, photodiodes, electro-optical modulators, optical fibers, switches, couplers, optical amplifiers, reflectors, filters, diffraction gratings, etc. There will usually be a preferred waveguide structure for each component, for technological or performance reasons, and the preferred structure is not always the same for all the components that need to be interconnected. Whence the need for optical waveguide transitions between different types of waveguide structure.
To enable industrial integration of optical or opto-electronic circuits, whether of the hybrid or monolithic type, it must be possible to fabricate the transitions between the various waveguide structures using the same technologies as are used to fabricate the waveguide structures themselves.
Various prior art monolithic or hybrid integration techniques are described in the following documents, for example:
D1: "Micro hybrid integrated devices and components: Micro Photonic Devices" by K. Imanaka, SPIE v.1751, Miniature and micro-optics, 1992. PA1 D2: "Monolithic vs. Hybrid approach for photonic circuits", by M. Erman, Proceedings ECIO, 1993. PA1 D3: "L'optique integree dans les materiaux semiconducteurs III-V", by A. Carenco et al., L'Echo des Recherches, no. 137, 1989. PA1 a) growing at least one epitaxial layer on a substrate; PA1 b) etching a waveguide layer to define a buried optical waveguide and a transition structure the width of which varies adiabatically; PA1 c) growing at least one top epitaxial layer on top of said buried optical waveguide and said transition area; PA1 d) etching a top layer to define a ridged optical waveguide structure superposed on said buried optical waveguide.
The prior art provides two techniques for integrating a buried optical waveguide and a ridged optical waveguide.
The first uses a butt-joint configuration, but this requires extremely close tolerances for the mechanical assembly which makes it difficult and costly but without which transmission losses at the transition are unacceptable.
The other technique is to use a mode converter at one end of the component, but this technique is extremely difficult if not impossible to integrate in a monolithic device.
The present invention attempts to remedy these drawbacks of the prior art and to facilitate the fabrication of an optical waveguide transition at reduced cost, with reduced transmission losses and in a manner compatible with integration.